I believe it's a hipster thing, some guys went "It'd really be cool and different for me to watch my little pony instead of its intended audience", I still believe that to be the case with most people that watch it, hence the name "Brony", to try and defend against others mocking the show rather than doing what people do with every other show, that is, ignore the people that don't like it and watch whatever the individual enjoys, but the natural insecurity about the subject matter prohibits Bronies from doing that, so this is the situation we end up with... Of course, that's just my theory.